The overall goal of this proposal is to define the molecular mechanisms by which osteoprotegerin (OPG) inhibits the progression of atherosclerosis. Growing evidence indicates that serum levels of OPG correlate with the severity of cardiovascular disease, and that OPG levels are an independent predictor of cardiovascular risk in humans. OPG, a secreted TNF-like decoy receptor for TRAIL and RANKL, plays a critical role in bone homeostasis and in chronic inflammatory diseases. We have shown that in mice prone to develop atherosclerotic lesions (ApoE-/- mice), OPG deficiency worsen the lesion burden, thus, suggesting that OPG contributes to limit lesion expansion and is an atheroprotective factor. Further, we have shown that OPG protects against lesion calcification in vivo, promotes smooth muscle cells (SMCs) survival, and induces MMP9 expression in SMCs and macrophages in vitro (Bennett et al. ATVB, 2006 Sep;26(9):2117-24). However, the molecular mechanisms underlying this new OPG effects are unknown. New preliminary data indicate that SMCs deficient for OPG are more sensitive to cell death and calcify more in response to high phosphate than wild type SMCs. We also find that the OPG ligand TRAIL increases OPG deficient SMC cell death and calcification while RANKL only exacerbates calcification but does not affect cell survival. In addition, inflammatory cells deficient for OPG express higher levels of inflammatory factors (TNF1 IL-12, IL-23, and CD86). Thus, it appears that endogenous OPG affects responses in SMCs and inflammatory cells. The aims of this proposal are: 1) to determine the mechanisms of OPG-mediated SMCs survival in vitro and whether SMC- specific OPG inhibits lesion progression; 2) to determine the mechanisms of OPG inhibition of SMCs mineralization in vitro and the role of SMC-specific OPG in lesion calcification; 3) to determine the phenotype of inflammatory cells in the vessel wall; whether the reintroduction of OPG in bone marrow-derived cell rescues lesion development; the role of OPG in the regulation of SMC cell death and calcification by inflammatory cells. PUBLIC HEALTH RELEVANCE: Cardiovascular disease, osteoporosis, and bone destructive inflammation often co-exist and they afflict especially the elderly and women. Osteoprotegerin is a very important regulator of bone tissues and clinical trials are underway to establish the potential osteoprotegerin-related therapies for bone disease. Our research is aimed at understanding whether the molecule osteoprotegerin may be also beneficial in cardiovascular disease. We believe that osteoprotegerin links the bone and cardiovascular systems. Thus our research may uncover new therapeutic approaches aimed to alleviate both bone and cardiovascular diseases.